Stereolithography (SL) is a known rapid prototyping technology which enables the generation of scale models of complicated three-dimensional parts in a fraction of the time and at a fraction of the cost of traditional methods. Generally, SL involves the use of electromagnetic radiation (e.g. a UV laser beam) to cure a photosensitive liquid (e.g. liquid photosensitive monomer or resin) which solidifies upon exposure to electromagnetic radiation of a given wavelength. When a layer is fully solidified upon exposure, the component stage drops down to allow a fresh layer of photosensitive liquid to flow over the solid surface. In this manner, a three-dimensional (3D) structure is fabricated from the bottom up, a layer at a time. SL provides a useful tool for visualizing components to assist in the iterative design process, as well for the direct fabrication of functional parts and microdevices.
Various stereolithographic methods are known for three-dimensional fabrication of microsystems. A first basic technique uses a scanning laser system to serially trace the shape of the desired part in a line-by-line manner over the free surface of a photosensitive resin bath. The laser is controlled by a CAD system that functions as an electronic mask, and typically allows for a transverse resolution of about 150 μm. In addition, the photopolymer can be loaded with ceramic, metal, or other particles to generate components of different materials. After initial stereolithographic fabrication, the parts can be sintered to remove the polymer and densify the functional material of interest. This usually shrinks the part by some controllable amount. An improvement on the scanning laser technique is known as the “Two Photon Absorption” method. This process uses two low power, pulsed laser beams which intersect deep within the resin bath. At the intersection point, the beams form a small volume which has sufficient photon flux to polymerize only the local material in the volume. While the beams can write a completely three-dimensional pattern into the resin bath, this is typically a slow process because it writes in a point-by-point fashion. Moreover, the types of resins available for this technique are severely limited due to the need that they be highly transparent to the laser beams, which also effectively prevents the loading of ceramic or metal particles in the resin bath.
Projection micro-stereolithography (PμSL) is a third, low cost, high throughput, micro-scale, stereolithography technique which projects a two dimensional image onto a photosensitive resin bath rather than a single spot, to fabricate complex three-dimensional microstructures in a bottom-up, layer-by-layer fashion. Originally, PμSL was first accomplished by using a set of photomasks to project the two-dimensional image. Although effective, this method requires a large number of photomasks thus limiting the practical number of layers possible. The use of a dynamically reconfigurable mask via a spatial light modulator (SLM) in PμSL systems dramatically reduced process time resulting in structures with thousand of layers. This was demonstrated in the form of a liquid crystal display (LCD) in the paper “Ceramic Microcomponents by Microstereolithography” by Bertsch et al (2004 IEEE). However, the LCD had some intrinsic drawbacks including large pixel sizes and low switching speeds.
The use of a Digital Micromirror Device (DMD, a trademark of Texas Instruments) as the SLM in a PμSL system is described in the paper “Projection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Mask” by C. Sun et al (2005 Elsevier). Similar to conventional SL techniques, PμSL with a SLM is capable of fabricating complex three-dimensional microstructures in a bottom-up, layer-by-layer fashion. A CAD model is first sliced into a series of closely spaced horizontal planes. These two-dimensional slices are digitized in the form of an electronic image and transmitted to the SLM. A UV lamp or LED illuminates the SLM which acts as a dynamically reconfigurable photomask and transmits the image through a reduction lens into a bath of photosensitive resin. The resin that is exposed to the UV light is cured and anchored to a platform and z-axis motion stage. The stage is then lowered a small increment and the next two-dimensional slice is projected into the resin and cured on top of the previously exposed structure. This layer-by-layer fabrication continues until the three-dimensional part is complete.
It is also known that imaging and lithography using conventional optical components is restricted by the diffraction limit. Features resolution in these systems is limited to one half of the wavelength of the incident light because they can only transmit the propagating components emanating from the source. It would be advantageous to provide an improved PμSL-based fabrication system and method capable of fabricating three-dimensional structures having sub-diffraction limited features, as well as other capabilities which enhance the resolution, materials flexibility, and process performance of standard PμSL.